A major focus of work is the study of CNS opioid receptors. A goal of this work is to further define opioid receptor subtypes and develop new small molecules as research tools. Using site-directed acylating agents and receptor autoradiography, the opioid receptors labeled by (-)-(3H) cyclo FOXY, an opiate antagonist suitable for positron emission tomography (PET), were shown to be Mu and not delta receptors. (+)Cyclo FOXY was shown not to interact with opioid, phencyclidine, or alpha receptors, supporting the use of 18 F-(+)-cycloFOXY to estimate "nonspecific" binding in PET studies. Using the site-directed alkylating agent, beta- funaltrexamine, we obtained autoradiographic data to support the existence of two Mu binding sites in rat brain. (3H)-6-beta- fluoro-6-deosxy-oxymorhone ((3H)-FOXY) was shown to label selectively and to be a high yield photoaffinity probe for Mu opioid receptors. The peptide, MeTyr-D-Ala-Gly-N(Et)-CH(CH2-N(CH3) (LY164929), was shown to be highly selective for the lower affinity (3H)D-ala2-D-leu 5 enkephin binding site. Nor-binaltorphimine, a selective k antagonist in vivo, was shown to be about 20-fold selective for k binding sites in vitro. An unexpected finding was the high levels of opioid peptides in rats brain membranes. Studies of morphine tolerance demonstrated that chronic morphine administration upregulated a Mu binding site labeled by (3H)cycloFOXY and that chronic morphine and chronic naltrexone upregulated opiate receptors by different mechanisms. Manipulation of fluid balance was shown to regulate physiologically neurophyophyseal k opiate receptors. Chronic drug abuse was shown not to alter psychotomimetic binding sites in human frontal cortex. Chronic administration of opiate agonists and antagonists and haloperidol were shown to alter rat brain phencyclidine (PCP) receptors. Studies of the interaction of enantiomeric pairs of unnatural opiates with PCP receptors led to the identification of (+)-pentazocine as a potential PET ligand for the delta receptor.